Generally, in metal joints in electrical and electronic devices, solder joints using solder pastes are used, and for such a solder paste, conventionally, a solder alloy containing lead is used.
However, recently, it has been required to suppress lead usage in view of environmental burden, and therefore a solder alloy (lead-free solder alloy) containing no lead is under development.
For such a lead-free solder alloy, for example, a tin-copper alloy, a tin-silver-copper alloy, a tin-bismuth alloy, and a tin-zinc alloy have been well-known, and in particular, a tin-silver-copper alloy has been widely used because of its excellent strength.
For example, Patent Document 1 (Ref: Examples 18 to 25) below has proposed such a tin-silver-copper solder alloy of, for example, a lead-free solder for on-vehicle electronic circuits, the lead-free solder containing 2.8 to 4 mass % of Ag, 3 to 5.5 mass % of In, 0.5 to 1.1 mass % of Cu, furthermore Bi, Ni, Co, Fe, P, Ge, and Zn, and a remaining portion of Sn.
Furthermore, Patent Document 2 (Ref: Examples 7 to 13) has proposed still another tin-silver-copper solder alloy: a lead-free solder for vehicles, the lead-free solder containing, for example, 2.8 to 4 mass % of Ag, 1.5 to 6 mass % of Bi, 0.8 to 1.2 mass % of Cu, and furthermore, Ni, Co, Fe, P, Ge, and In, and a remaining portion of Sn.